The invention relates to a bearing assembly for a rotary screw gas machine. A typical rotary screw machine has a housing with two axially extending intersecting bores, a low pressure connection at one end a high pressure connection at the other end. Two rotors with complementary intersecting spiral screws are rotatable in the bores and are carried by bearings mounted in the end walls of the housing. Typically, it is the bearings of a rotary screw machine which are the first component to fail in normal operation. Bearing size is limited by the center-to-center spacing of the rotors, and cannot be increased to provide a heavier bearing. Accordingly, it is desirable that the thrust loads be distributed between the bearings at the low pressure end and high pressure end and the appropriate bearing characteristics be selected for maximum bearing life.
The most common rotary screw gas machine is the gas compressor, widely used for compressing air and in refrigeration systems. Much of the following discussion will be concerned with the machine as a gas or air compressor. The novel bearing assembly may, however, be used with other rotary screw gas machines having a pressure differential between the inlet and outlet.
The radial bearing load at the low pressure end of the machine is relatively small as the fluid between the rotors is at a low pressure. In an air compressor, the pressure at the inlet is atmospheric. Conversely, at the high pressure end, the fluid trapped between the rotors is at high pressure and there is a substantial radial load on the high pressure end bearings. The high pressure fluid at the high pressure end is trapped between the end faces of the rotors and the end wall of the housing. This establishes a substantial axial force on the rotors. It is desirable that the rotors be fixed axially in the housing so that the end faces of the rotors have a substantially fixed clearance from the end wall at the high pressure end of the housing. A variable clearance, as that necessary to accommodate thermal expansion of the rotors, is provided at the low pressure end of the housing. This relationship requires that the bearings at the high pressure end support a large thrust load in addition to the large radial load.